String searching and extraction in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What method in C# can you use to find the first position of a substring within a string?

You can use the IndexOf() method. It returns the zero-based index of the first occurrence of the substring, or -1 if not found.

beginner

How do you extract a part of a string in C#?

Use the Substring(startIndex, length) method. It returns a new string starting at startIndex and continuing for length characters.

intermediate

What does LastIndexOf() do in string searching?

It finds the last occurrence of a substring in a string and returns its index. If the substring is not found, it returns -1.

beginner

How can you check if a string contains a certain substring in C#?

Use the Contains() method. It returns true if the substring is found anywhere in the string, otherwise false.

intermediate

What happens if you use Substring() with a startIndex that is out of the string's range?

It throws an ArgumentOutOfRangeException. Always ensure startIndex and length are within the string's bounds.

Which method returns the index of the first occurrence of a substring in C#?

A Contains()

B IndexOf()

C Substring()

D LastIndexOf()

IndexOf() returns the position of the first match, while Contains() returns a boolean.

What does Substring(3, 4) do on the string "HelloWorld"?

A "oWol"

B loWo"

C "loWo"

D "loWo

Starting at index 3 (0-based), it extracts 4 characters: 'l', 'o', 'W', 'o'.

If IndexOf() returns -1, what does it mean?

A Substring was not found

B Substring is empty

C Substring was found at the start

D Substring is at the end

-1 means the substring does not exist in the string.

Which method checks if a string contains another string and returns true or false?

A LastIndexOf()

B Substring()

C IndexOf()

D Contains()

Contains() returns a boolean indicating presence of the substring.

What exception is thrown if Substring() is called with an invalid index?

A ArgumentOutOfRangeException

B InvalidOperationException

C IndexOutOfRangeException

D NullReferenceException

ArgumentOutOfRangeException is thrown when indexes are outside the string's range.

Explain how to find and extract a substring from a string in C#.

Think about first locating the substring, then extracting it.

Describe what happens if you try to extract a substring with invalid indexes in C#.

Consider what the program does when indexes are outside the string length.

Practice

1. What does the IndexOf method return if the searched substring is not found in a string?

easy

A. 0

B. -1

C. null

D. An exception is thrown

Solution

1. Step 1: Understand IndexOf behavior

   The IndexOf method returns the zero-based index of the first occurrence of the substring if found.

2. Step 2: Check return value when substring is missing

   If the substring is not found, IndexOf returns -1 to indicate absence.

3. Final Answer:

   -1 -> Option B

4. Quick Check:

   IndexOf returns -1 if substring missing [OK]

Hint: Remember: Not found means -1 from IndexOf [OK]

Common Mistakes:

• Thinking it returns 0 when not found
• Expecting null instead of -1
• Assuming it throws an error if missing

2. Which of the following is the correct syntax to extract a substring starting at index 3 with length 5 from a string text?

easy

A. text.Substring(3, 5);

B. text.SubString(5, 3);

C. text.Substring(5);

D. text.Substr(3, 5);

Solution

1. Step 1: Recall Substring method signature

   Substring takes start index first, then length: Substring(int startIndex, int length).

2. Step 2: Match correct syntax

   text.Substring(3, 5); uses correct method name and parameter order: start at 3, length 5.

3. Final Answer:

   text.Substring(3, 5); -> Option A

4. Quick Check:

   Correct method and parameters = text.Substring(3, 5); [OK]

Hint: Remember: Substring(startIndex, length) with capital S [OK]

Common Mistakes:

• Using wrong method name like Substr or SubString
• Swapping start index and length parameters
• Using only one parameter when two are needed

3. What is the output of this code?

string s = "hello world";
int pos = s.IndexOf("world");
string part = s.Substring(pos, 5);
Console.WriteLine(part);

medium

A. hello

B. worldd

C. world

D. error

Solution

1. Step 1: Find index of "world" in string

   "world" starts at index 6 in "hello world".

2. Step 2: Extract substring from index 6 with length 5

   Substring(6, 5) extracts "world" exactly.

3. Final Answer:

   world -> Option C

4. Quick Check:

   IndexOf + Substring extracts "world" [OK]

Hint: IndexOf finds start, Substring extracts exact length [OK]

Common Mistakes:

• Using wrong start index for Substring
• Confusing length parameter
• Expecting output to include extra characters

4. The following code throws an exception. What is the main cause?

string s = "example";
int pos = s.IndexOf("z");
string part = s.Substring(pos, 3);
Console.WriteLine(part);

medium

A. Substring called with negative start index

B. IndexOf throws exception if not found

C. Length parameter is too large

D. Console.WriteLine cannot print substrings

Solution

1. Step 1: Check IndexOf result for "z"

   "z" is not in "example", so IndexOf returns -1.

2. Step 2: Substring called with start index -1 causes exception

   Substring cannot start at negative index, so it throws ArgumentOutOfRangeException.

3. Final Answer:

   Substring called with negative start index -> Option A

4. Quick Check:

   Negative index in Substring causes error [OK]

Hint: Check if IndexOf is -1 before using Substring [OK]

Common Mistakes:

• Assuming IndexOf throws exception when not found
• Ignoring negative index causes Substring error
• Blaming Console.WriteLine for error

5. You want to extract the first word from a sentence stored in string sentence. Which code correctly extracts the first word assuming words are separated by spaces?

hard

A. string firstWord = sentence.Substring(sentence.IndexOf(' ') + 1);

B. string firstWord = sentence.Substring(0, sentence.IndexOf(' '));

C. int spacePos = sentence.IndexOf(' ');
string firstWord = sentence.Substring(spacePos);

D. int spacePos = sentence.IndexOf(' ');
string firstWord = spacePos == -1 ? sentence : sentence.Substring(0, spacePos);

Solution

1. Step 1: Find position of first space

   Use IndexOf(' ') to find where the first space is.

2. Step 2: Extract substring from start to space or whole string if no space

   If no space found (-1), the whole sentence is one word; else extract from 0 to space position.

3. Final Answer:

   int spacePos = sentence.IndexOf(' '); string firstWord = spacePos == -1 ? sentence : sentence.Substring(0, spacePos); -> Option D

4. Quick Check:

   Check for space, then substring from start [OK]

Hint: Check if space exists before substring to avoid errors [OK]

Common Mistakes:

• Not handling case when no space exists
• Extracting substring after space instead of before
• Using wrong substring parameters